black face tweed

[stingray_65]

ok,
I'm gathering parts for my next build.
I've salvaged a PT and OT, sockets, a bunch of 12AX7's, a tag board, a few different rectifier tubes (3 5Y3GT's and a 5V4), a hand full of switchcraft 12A jacks, a chassis 12 x 9 x 3 1/2 and some other odds n ends.

before I go too hedlong into purchasing specific valued components I figured I should get the opinions of a few people who know more than I do.

I loved the tone of the 5E3 I built, especially after I got some decent tubes.
I did not like the almost random interactions of the 2 volume controls and the tone control. Navigating the controls is a bit too much like using a Ouji Board.

so I read up best I could and I basicly "cut and paste" a black face single channel input with tone stack to a 5E5 pro PI and power section.

attached is the schematic I bastardized and I have a few questions:

1st have I done something horribly wrong?

2nd is that 5V4 recto tube enough for this circuit? do I need a stiffer recto, something to give me more current capacity?
I'm aware that I simply copied the values for the B+ resistors and will likely have to change them to bring the B+ voltages in line.

I've seen fuses on the B+ rail on other amps before the first cap. Is this a good idea to do? or leave things well enough alone?

I would like to put a line out, to patch into a PA board.
Can I use the 500 ohm tap on the OT for this? is this advisable if I can?

I guess that's my questions for tonight, I'm sure I'll have more soon.

I've also opted to use 6BG6GA for the power tubes. I've read they are identical to 6L6's except for the pin out and the plate cap on top, which I thought would look good being it is an open chassis and will be visble.

[black_labb]

on a quick glance it seems alright. a fuse on the b+ is probably a good idea to protect things. not entirely neccesary, but keeps things safer.

[txstrat]

I think the circuit like you've drawn it wouldn't give you a "tweed tone".
Basically it's sort of a Champ II circuit (http://www.schematicheaven.com/fenderamps/champ_II.pdf), which has a third gain stage after the tone stack for gain options.
In your schematic V2A is not part of the PI but the third gain stage.
If you like to leave V2A in the circuit I would add a coupling cap between the plate of V1B and V2A (like .005uF or .01uF or so).
To get "sort of a" tweed like tone from this circuit you could add a .002uF cap parallel to the 220pF cap in you tone stack. You might make it switchable for more versatility (that's what the mid boost in the champ II circuit does).

The 5U4G recto (like in a 5e5) provides 220ma DC current while the 5V4G only provides 175ma. Don't know what your PT will be so it's up to you to determine which is the right one.

The fuse in the B+ rail is not essential for the circuit. I usually add it for safety reasons. Others say it's not necessary, though.

The question regarding the 500ohms tap is ahaed of my skills level.
May the pro's chime in?

[Steve Conner]

The 500 ohm tap may well be to drive a 70 or 100 volt line for PA applications. That's enough voltage to fry a mixing board.

You could attenuate it, but there's no point in using it if it's just a tap on the speaker winding. It would only be worth using if it were an isolated winding, so you could get a floating output to break ground loops.

[Robert M. Martinelli]

Seems like our "fellow solder fume breathers" ( credits : Enzo ) have already pointed you in the right direction, I would only like to add a small tip about safety, I wouldn't use 6BG6s or, more in general, any output tube with "plate caps", all the more so in an "open chassis" amp....I don't feel comfortable at the idea of having 400-some volts at my ( or someone's else ) fingertips....That's not my idea of "going electric"

Hope this helps

Best regards

Bob

[stingray_65]

I added the coupling cap and "tone" cap and switch as per your suggestions TXSTRAT, thank you.

I'm still kind of stuck on the idea of a line out.

Would it be better to pull the line out off the 4 ohm tap instead of the 500 like this?

Would adding an audio isolation transformer be beneficial?

If so what suggestions might you have? A small torroid?

any suggestions here would be apreciated.

And BOB, thank you for your concern I truly appreciate your advice and will consider a way to shield access to the plate caps. Those 6bg6's are already in my possession and my goal is to build a medium powered great sounding amp for less than $100. If it is any consolation to you this is going to be an amp head and none of the tubes will be upside down. All of the inputs, controls and outputs will be on the front panel so there would be no need to reach over or around the chassis except to get to the mains in or fuses.

SO!
if the circuit itself seems ok (with the exception of ironing out the line out) I can go ahead and start working on the layout.

On a side note
Thank you everyone for your input. You all make it possible for me to enjoy making these sorts of things. It's an important and affordable outlet for me while I weather out this economic downturn. My son and his friends take an interest in this and it's nice to have them watch the progress and express an eagerness for the results. In short you guys are helping me keep my sanity and sense of self worth. My entire family appreciates that I have something that keeps me amused and focused. Mostly I'm thankful that I never have to watch another airing of "Oprah" because I haven't got something better to do LOL.

RAY

[txstrat]

Mostly I'm thankful that I never have to watch another airing of "Oprah" because I haven't got something better to do LOL

That's a welcome side effect, ain't it?
Would be nice if you post the result or the progress in here.

cheers

Matt

[stingray_65]

I'll do just that.
I can take lots of pics and if the line out works I'll even record a few samples.
I've been wishing for a nice Shure sm57 to mic a few of these amps i've done, but I can probally come up with somthin.

just for fun I can even keep a running tab on the parts.